Electrolytic reactor comprising a cathode and an anode

ABSTRACT

The invention concerns an electrolytic reactor, in particular for separating phosphate from phosphate-containing liquids and recovering phosphate salts, comprising a housing, an inlet and an outlet for the liquid and two electrodes of different polarity, which enclose a reactor chamber between them, whereby at least one of the two electrodes is a sacrificial electrode, whereby between the inlet and the reaction chamber a pre-chamber is provided in which the inserts are arranged such that the inlet stream is divided by the inserts into two partial streams and directed around the inserts.

BACKGROUND OF THE INVENTION

In process engineering, electrolytic reactors comprising one cathode and one anode are often used. When the reactor is operating, an electrical voltage is applied between the cathode and the anode such that the anode is consumed (sacrificial anode).

DE 10 2010 050 691 B3 and DE 10 2010 050 692 B3 describe a method and a reactor for recovering phosphate salts from a liquid whereby the sacrificial electrodes consist of a magnesium-containing material.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an electrolytic reactor, in particular for separating phosphate from phosphate-containing liquids and for recovering phosphate salts, comprising a housing, an inlet and an outlet for the liquid and two electrodes of different polarity which enclose a reaction space between them, whereby at least one of the two electrodes is a sacrificial electrode.

With such reactors, optimal conversion and reaction is always desired whereby the optimal conversion rate can be achieved with a flow control in the reactor that is as constant as possible. It is therefore the object of the invention to provide a reactor that has the most constant flow possible throughout the entire reactor.

The object is achieved by a reactor of the kind mentioned above, in which a pre-chamber is arranged between the inlet and the reaction chamber in which inserts are arranged such that the inlet stream through the inserts is divided into two partial streams directed around the inserts. It is particularly advantageous when a corresponding point can be assigned to each point in the cross section of the inlet profile, namely the entry cross section of the reactor chamber whereby a partial flow path can be defined as the connection between the points, and the sum of the flow resistances is the same in each partial flow path. This can have the effect that the same flow velocity applies over the entire width of the reactor whereby the same dwell time applies to all the inflowing liquid in the reaction chamber, and thus an equal rate of reaction can take place. In particular, the pre-chamber also has the effect that the flow rate of the inlet is adapted from a first stream cross-section to the stream cross-section of the reaction chamber.

In addition, thanks to the equal conversion, when phosphate or other materials are to be separated in the form of salt, an equitable crystal growth is to be achieved, and the resulting distribution of crystal grain sizes is concentrated around a narrow size spectrum, such that the effort of subsequent segregation of crystals with different traditional means such as centrifuging, sedimentation and the like can be minimized. In particular, such crystals whose grain size is as homogeneous as possible can be particularly easily processed further, for example into fertilizer, provided that they are phosphate crystals. In addition, an even crystal growth has the advantage that the crystals formed by means of the electrolytic reaction can be discharged evenly with a defined flow force.

It is particularly preferred when the inserts consist of one or more bulkheads. In vertical position, the inserts fill the entire pre-chamber such that all the liquid must be directed around the inserts.

It is also possible—in addition to an appropriate pre-chamber—to provide an appropriate after-chamber between the inlet and the reaction chamber with which the stream between the outlet and the reaction chamber is also controlled, to achieve an even outlet of the liquid although the stream cross-section changes from the reaction chamber to the outlet. It is especially preferred when the after-chamber also has inserts, preferably in the form of bulkheads.

It can be provided in particular that the inlet and/or the outlet has a circular stream cross-section and the reaction chamber has a rectangular stream cross-section. In particular it can be provided that the stream cross-section of the reaction chamber is much wider than high. Preferably the ratio is at least 1:50, more preferably at least 1:70 and still more preferably 1:100. In addition, the reactor's ratio of height to length—with the length meaning the reactor's extension in liquid flow direction—is to be at least 1:50, more preferably at least 1:70, more preferably at least 1:100 and still more preferably at least 1:150. It is important that the stream is already distributed across the entire reactor width when it enters the reaction chamber, such that it can provide conversion and reaction across all the electrodes which enclose the reactor chamber between themselves.

In particular it can be provided that the inserts are at least wider than the width of the reaction chamber. In particular, the inserts protrude on each side by at least a length C beyond the width of the reaction chamber, whereby the length C corresponds at least to the width D of the flow path between the inserts and the wall of the pre-chamber or the after-chamber which is facing the reaction chamber. However, preferably the length C is greater, preferably 1.5 times greater, in particular 2 times greater than the width D.

Furthermore it is preferred that the distance between the inserts and the reaction chamber is at least one tenth of the width of the reaction chamber.

It can also be especially preferred that at least on of the two electrodes, preferably the top electrode in the direction of use, is movable relative to the lower electrode to hold the reaction chamber constant in height when the at least one sacrificial electrode is used up. When the at least one sacrificial electrode is consumed, this has the effect that when the electrodes are installed in a housing, the distance between the upper electrode and the upper housing part becomes greater unless the upper housing part is not carried along when the upper electrode is moved. In that case it can be preferably provided that there is a separation between the upper electrode and the upper housing, which separation is such that it prevents liquid from entering the section above the upper electrode and that all the liquid enters the reaction chamber, and it is formed such that the dividing wall can follow the consumption of the sacrificial electrode and thus the movement of the movable electrode. In particular, a kind of flexible flat material sheet can be used for that which is first folded in the manner of an accordion at the starting time of the reactor and which appropriately unfolds as the electrode is consumed. For example, a flexible rubber or plastic film can be provided here or a coated fabric or an articulated plate. In this way, the entering liquid is directed into the reaction chamber in any case. The formed crystals are removed safely, and the plugging or blocking of the reaction chamber is prevented. To improve the exchangeability of the electrodes in the housing, the covering on the electrode can for example be fastened with a weight being placed upon it.

In general, if the electrodes are made of a material containing magnesium, it is provided that the following reaction occurs while phosphorus is being converted:

Mg²⁺+NH₄ ⁺+PO₄ ³⁻+6H₂O→MgNH₄PO₄-6H₂O,

where the magnesium ions are liberated at the surface of a sacrificial magnesium anode. Here, the electrodes lead to the electrolytic recovery of phosphorus in the form of crystallized magnesium ammonium phosphate (MAP/Struvite) with magnesium deficiency in the initial substrate.

If one of the two electrodes or both electrodes are consumed during the reaction, it is advantageous when—as described—the distance between the electrodes always remains the same, and if therefore the electrical field between anode and cathode always remains the same, and optimal conversion rates can be achieved. It is advantageous when none of the two electrodes is permanently used as a cathode or anode, but if a pole reversal can occur at certain intervals. Without pole reversal, deposits can form on the cathode. Thanks to pole reversal, these deposits are dissolved together with the consumption of the anode and can be removed from the reactor with the liquid stream. When the invention does not provide that both electrodes are meant to be sacrificial electrodes, it is preferred that the cathode which is not consumed is made of stainless steel or another noncorrosive electrically conductive material.

It is of special importance for process control that there is a constant spacing between the surfaces of anode and cathode independently of the consumption of the sacrificial electrode. Preferably, the surface which limits the reaction chamber through which the liquid to be treated is flowing, is level whereby the reaction chamber preferably has a rectangular cross section and the electrodes have a cubic shape or in top view have a rectangular shape which does not change significantly due to consumption. Thanks to the constant geometry of the reaction chamber, the electrical field is also held constant while the electrode is being consumed, and defined and high conversion rates can be achieved with a minimum consumption of energy.

For example, tracking of the one electrode to the other electrode can be done by means of gravity, with one or two springs and/or one or more actuators. When gravity is used, for example to track an upper electrode to a lower electrode, either springs or electric, pneumatic or hydraulic actuators can be used in addition to gravity, whereby it is preferred to install two spacers at a distance from each other to prevent the tilting of the electrodes against each other.

In addition, it is also possible, in particular when actuators are used for tracking an electrode, to regulate or control the spacing between the electrodes and to detect the distance with sensors which measure the consumption or the remaining thickness of one or both electrodes as part of a control circuit. Such sensors are commercially available.

It is particularly preferred for the surfaces of the electrodes bordering the reaction chamber to be level, whereby the electrodes must have a certain thickness if they are to be used as sacrificial electrodes such that in case of a rectangular surface facing the reaction chamber, the unused electrode will have an essentially cubic shape.

In particular it is an advantage when the external dimensions (length and width of the housing) of the inventive reactor agree with the dimensions of customary transportation means such as the so-called Europallets. In that case the housing base can be used as a multi-use transportation container and transported at a reasonable cost in existing logistics chains.

It is also possible to provide means to detect the position of the electrodes, to gauge the process taking place in the reactor. For example, these can be position sensors of any design. Preferably they can be movably fastened to the housing of the reactor or to the electrode. This is how the consumption of the electrodes can be monitored in a simple and very reliable manner.

Finally, means are provided to detect the electrical current flowing between the electrodes and/or the voltage applied between the electrodes. With this, the process taking place in the reactor can be monitored simply and reliably. Malfunctions of the process lead to a change in the electrical current and/or voltage and can thus be simply detected.

It is especially preferred when the sacrificial electrode, preferably also both electrodes, are made of a magnesium-containing material. This may also include more or less pure magnesium as electrode material. If only one electrode is provided as a sacrificial electrode, the second electrode can be made of stainless steel which is electrically conductive and is not affected by the liquid to be treated in the reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and advantageous embodiments of the invention are shown in the figures below where:

FIG. 1 shows an inventive reactor in sectional view, and

FIG. 2 shows an inventive reactor in sectional top view.

DETAILED DESCRIPTION

FIG. 1 shows a reactor 10 with a housing 12 comprising a housing base 14 and a housing top 16. The reactor also comprises an inlet 18 and an outlet 20. The flow direction is marked by an arrow 22. Arranged in the housing, which is closed by the housing top 16, here shown as a lid, are two electrodes 24 and 26, both designed as sacrificial electrodes, and both serving in alteration at different time intervals as anode and as cathode. Both electrodes 24 and 26 consist of a magnesium-containing material and are consumed in the course of the reaction when phosphorus is converted with the phosphate into MAP from the liquid. Between the two electrodes 24 and 26 is a reaction chamber 28 designed as a spacing whereby the length L of the reaction chamber is much greater than the height of spacing S. In particular the spacing height S is also much smaller than the width B of the electrodes, as shown in FIG. 2. In particular, a height/length ratio of 1:150 and a height/width ratio of at least 1:100 is provided. Electrode 26 is movable in the housing and can be tracked to electrode 24 such that the spacing height S always remains constant, even when electrodes 24 and 26 are consumed.

To equalize the liquid stream in reaction chamber 28, a pre-chamber 30 and an after-chamber 32 are provided, each with an Insert 34, 36. These inserts are bulkheads or dividing walls with the purpose of diverting the liquid entering reactor 10 from Inlet 18 in a plane vertical to the plane of the drawing in FIG. 1, such that the liquid has to flow around dividing walls 34 and 36. In the same manner, an after-chamber 32 is provided in the outlet region in which the liquid is diverted such that it can enter outlet 20 with especially good fluidic properties. This is to accomplish that the stream in the entire reactor 28 is as even as possible and that entering fluid remains in the reaction chamber for about the same time. This improves the transfer from a stream of circular cross-section, as with Inlet 18, to a stream with a flat cross-section, as that of the reaction chamber, and then again to the outlet stream with a circular cross-section.

In addition, as FIG. 1 shows, a covering 40 is provided on the upper electrode 26 which can be rigidly connected with the housing lid 16 and is attached to the upper electrode 26, for example with a weight 42. If the upper electrode is moved for tracking on account of its consumption, the spacing—identified in FIG. 1 by the letter a—becomes larger. Thus, the risk increases that liquid flows across the electrodes instead of through reaction chamber 28. This can be prevented with covering 40, which in particular can be a flexible film, such that the tracking of electrode 26 is possible without a problem.

FIG. 2 shows reactor 20 in sectional top view. The section plane runs through reaction spacing 28. Liquid which enters in inlet 18 is diverted to the right in the area of pre-chamber 30 and to the left in the direction of the drawing, and flows around a dividing wall 34. This dividing wall 34 is arranged at a distance D before wall 31 that is facing pre-chamber 30. Bulkhead 34 protrudes by at least one length C on both sides beyond the width of reaction chamber 28 in the entry section thereof, whereby width C corresponds to at least width D, with the stream being directed such that the liquid enters the reaction chamber with an even flow geometry distributed across the entire reaction chamber. In the embodiment, the entry cross-section is slightly smaller than width B of the electrodes. It can thus be achieved that for every point in the inlet region there is a defined point in the entry region to reaction chamber 28, and the points can be connected via a partial flow path, whereby the sum of all stream resistances of the partial flow path is always the same.

If the flow enters the reactor with as even a stream cross-section as possible and as simultaneously as possible, particularly good reaction rates can be achieved in the reactor, and—as mentioned above—crystals can be produced with as even a grain size as possible, which facilitates the later separation and further processing. This measure can be further improved when in the outlet region 20 between reaction chamber 28 and outlet 20 an after-chamber 32 is interposed in which a bulkhead 36 is provided around which the liquid flows such that it can be directed through the outlet with circular cross-section. Preferably, the selected distances D and C should be equal. In the above manner, the throughput rates can be optimized.

The dividing walls 34, 36 are arranged in the pre-chamber and after-chamber such that when they are installed, the liquid can only flow past them on both sides, but not above or below and through them. 

What is claimed is:
 1. Electrolytic reactor, in particular for separating phosphate from phosphate-containing liquids and recovering phosphate salts, comprising a housing, an inlet and an outlet for the liquid and two electrodes of different polarity which enclose a reactor chamber between them, whereby at least one of the two electrodes is a sacrificial electrode, characterized in that between the inlet the reaction chamber a pre-chamber is provided in which inserts are arranged such that the inlet stream is divided by the inserts into two partial streams and directed around the inserts.
 2. Reactor according to claim 1, characterized in that between the reaction chamber and the outlet an after-chamber is provided in which the inserts are arranged such that the outlet stream is divided into two partial streams by the inserts and directed around the inserts.
 3. Reactor according to claim 1, characterized in that the inserts consist of one or more bulkheads.
 4. Reactor according to claim 1, characterized in that the inlet and/or the outlet have a circular stream cross-section and the reaction chamber has a rectangular stream cross-section.
 5. Reactor according to claim 1, characterized in that the distance between the inserts and the reaction chamber is at least one tenth of the width of the reaction chamber.
 6. Reactor according to claim 1, characterized in that die inserts on both sides are wider by at least the length C than an entry cross-section of the reaction chamber.
 7. Reactor according to claim 1, characterized in that the stream cross-section of the reaction chamber is much wider than it is high, in particular that the height to with ratio is at least 1:50, preferably at least 1:70 and more preferably at least 1:100.
 8. Reactor according to claim 1, characterized in that the reaction chamber has a rectangular cross-section in flow direction and a constant stream cross-section throughout the entire reaction chamber.
 9. Reactor according to claim 1, characterized in that the electrode which is on top during operation is movable and that the bottom electrode can be tracked to maintain a constant height (S) of the reaction chamber.
 10. Reactor according to claim 1, characterized in that between the housing, preferably between a housing half in the operating state, and the electrode which is at the top in the operating state, a covering is provided which prevents liquid from entering that region.
 11. Reactor according to claim 10, characterized in that the covering consists of a flexible material and is arranged such that a movement of the top electrode can be adjusted by a movement thereof. 